Endoplasmic reticulum (ER) stress and major chemotherapeutic agents damage DNA by generating reactive oxygen species (ROS). Here we show that ER stress and chemotherapy induce leukotriene C4 (LTC4) biosynthesis by transcriptionally upregulating and activating the enzyme microsomal glutathione-S-transferase 2 (MGST2) in cells of non-haematopoietic lineage. ER stress and chemotherapy also trigger nuclear translocation of the two LTC4 receptors. Acting in an intracrine manner, LTC4 then elicits nuclear translocation of NADPH oxidase 4 (NOX4), ROS accumulation and oxidative DNA damage. Mgst2 deficiency, RNAi and LTC4 receptor antagonists abolish ER stress- and chemotherapy-induced ROS and oxidative DNA damage in vitro and in mouse kidneys. Cell death and mouse morbidity are also significantly attenuated. Hence, MGST2-generated LTC4 is a major mediator of ER stress- and chemotherapy-triggered oxidative stress and oxidative DNA damage. LTC4 inhibitors, commonly used for asthma, could find broad clinical use in major human pathologies associated with ER stress-activated NOX4.